How many host addresses are available on the mobile network using this subnet mask?

I need help to discover how to identify how many host addresses are available on the mobile network using this subnet mask 255.128.0.0.

I know the answer is 2^23-2 = 8388608 (8388606 - less subnet address and broadcast address)

But why is it this answer. The mobile network is a class A address that has 2^24-2 host addresses. So would the number of hosts on the subnet mask always be 1 less, is this why it is 23?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).